DC to DC converters are essentially built up around a transformer having two primary windings mounted in opposition and each connected in turn to a constant voltage source via appropriate switching means.
On each alternation, such switching, which may optionally be the result of self-oscillation in the circuit, produces a corresponding reversal of the magnetic flux in the transformer, thereby making it possible to take an alternating voltage from a secondary winding of the transformer, which alternating voltage is then rectified and filtered in appropriate manner.
In such converters, the size (and thus the weight) of the apparatus is reduced by increasing the operating frequency at which the primary windings are switched in alternation. When attempts are made to increase this operating frequency, it becomes particularly advantageous to use MOSFET type transistors as the switching means because of their excellent switching performance at high frequency.
However, these components, in particular high power MOSFETs, have stray capacitance which is far from negligible, both between drain and source and between drain and grid. That is why the switching cycle used with such components must take account of the charging and discharging times of these stray capacitances at the switching rate.
Since such alternating charging and discharging is inevitable, it is particularly important to design the circuit of the converter so that in spite of this phenomenon switching takes place without losses and with as little degradation as possible of the form factor, which parameter governs the operating efficiency of the converter.
In this respect, although it is possible to ensure that the charging and the discharging of the stray capacitances of the MOSFETs takes place without losses, it is also essential for the charging and discharging times (which correspond to a transition or "switching interval" stage during which the converter is not transferring power) should be adjusted to the selected switching frequency.
Thus, taking a push-pull converter operating at 200 kHz as an example, the maximum conduction time is 2.5 .mu.s in each leg, in which case it is not acceptable for the switching interval to exceed 1 .mu.s since the conduction time of the transistors would then become too short, thereby excessively degrading the form factor.
This problem is even more critical when MOSFETs are used as the switching means since under such circumstances the capacitance of the MOSFETs predominates over the stray capacitance of the transformer, which means that it is not possible to determine the duration of the switching interval by the characteristics of the transformer (reference may be made in this respect to U.S. Pat. No. 4,443,840 which describes a converter using conventional transistors and the intrinsic resonance of the transformer and the output rectifier to adjust the duration of the switching interval to the operating frequency, which could not be done using MOSFETs).
One of the objects of the present invention is to provide means for adjusting the duration of the switching interval to the operating frequency in a converter where the capacitance of the switching means is greater than the capacitance of the transformer, with this typically applying to converters in which switching is performed by MOSFETs.
A converter of this type is described, for example, in FR-A-2 627 644 in the name of the present Applicant. This document describes a switching mode according to which the stray capacitance of the MOSFETs is taken into account, but it does not in any way suggest adjusting the duration of the switching interval as a function of the selected switching frequency.
In addition, as described below, the means proposed by the present invention for adjusting the duration of the switching interval are independent of the circuit diagram and of the various electrical parameters concerned, and can therefore be applied equally well to resonant type converters (i.e. those that operate by spontaneous oscillation) and to non-resonant converters (i.e. those operating by forced oscillation), and similarly it can be applied equally well to current-controlled converters and to voltage-controlled converters.